Asynchronous Transfer Mode (ATM) or “cell switching” is a method of transmitting digital information wherein the information is broken into equal size units named “cells”. Each cell is 53 octets in length that consists of 5 octets of cell header information and 48 octets of payload. Using the portion of the cell header known as the virtual path index (VPI) and the Virtual Connection Index (VCI) multiple streams of user data can be carried over a single ATM physical media. Most ATM switches support a wide range of media types for digital transmission such as T1, E1, DS-3, E-3, OC-3, OC-12, etc. Regardless of media type, each media interface within a given ATM switch is identified uniquely and thus each user stream entering or leaving the switch can be identified uniquely using the following naming convention, <interface_index.virtual_path_index.virtual_channel_index>. Each unique user stream within a given media interface is known as a connection segment. A complete ATM connection is the concatenation of one or more ATM connection segments. The act of concatenating the connection segments is the primary function of an ATM switch. There are two forms of concatenation, a unicast cross-connection and a multicast cross-connection. A unicast cross connection is described using the following naming pair, <<upstream:interface_index.virtual_path_index.virtual_channel_index>,<downstream:interface_index.virtual_path_index.virtual_channel_index>>. A multicast cross-connect differs slightly from the above example in that there can be more than one downstream connection segment.
Voice transport over any network technology poses a unique set of problems or requirements for the underlying networking technology. Voice is extremely intolerant of delay variation. Excessive “jitter” in the form of queuing delays or payload disordering render it difficult to accurately recreate the voice conversation at the receiving end. Excessive total delay caused by cumulative switching delays and large sampling intervals can lead to excessive total delay resulting in excessive echo. Loss of voice payloads is tolerated providing the loss detection techniques do not introduce increased “jitter”. Similar to ATM, an ideal solution would divide voice and data into uniform chunks for carriage across the core of the network and introduce queuing mechanisms to ensure constant rate delivery of the voice payloads. One issue with the use of ATM for the transport of voice is the size of the payload. Prior to the advent of ATM AAL2 the predominant techniques for voice transmission, namely AAL1 and AAL5 proved to be either wasteful of network bandwidth because most voice payloads are much smaller than 48 octets or wasteful of bandwidth because of excessive header and trailer overhead. These issues are particularly exacerbated over low speed ATM access such as DSL or T1/E1 transmission medium. For example, a T1 interface running TDM can accommodate 24 channels of uncompressed voice whereas 24 channels of uncompressed voice over ATM will not fit in a T1 cell interface.
These issues lead to the development of ATM Adaptation Layer 2(AAL2). In AAL2, another layer of stream identification known as the Channel Identifier (CID) was introduced and a format devised to allow the packing of multiple voice streams sharing a common destination into a single ATM connection. Using this technique a voice stream is uniquely identified using the following naming convention, <interface_index.virtual_path_index.virtual_channel_index.channel_id>. This opens the opportunity to provide a switching function for voice flows analogous to ATM cell switching such that a voice connection could consist of one or more concatenated voice cross-connects, described as follows, <<upstream: interface_index.virtual_path_index.virtual_channel_index.channel_id>,<downstream:interface_index.virtual_path_index.virtual_channel_index.channel_id>>. AAL2 provides for the setup of ATM AAL2 connections that are configured to support up to 255 voice channels carried within or can be restricted to just one voice channel carried within. AAL2 connections can be configured as permanent virtual circuits by the operator or they can also be set up on demand as switched virtual circuits using the ATM networking layer. Given that AAL2 implements a voice network on top of an ATM network infra-structure using an overlay technique, it is natural to expect that the configuration of multiplexed connections between any two voice switching nodes involves some network engineering analysis by the operator. Inevitably this analysis will raise questions regarding how much bandwidth to assign to the multiplexed connections, what action should be taken when bandwidth is exhausted on a multiplexed connection and how to promote efficient bandwidth utilization within these connections. An over-commitment of bandwidth can reduce network bandwidth efficiency and an under-commitment can deny service to users, particularly at peak times. Particularly, in cases where the underlying ATM network has plenty of unused capacity, service denial constitutes an error and perhaps a failure to honor the service level agreement (SLA) between the network operator and the customer.